In WDM (wavelength division multiplexed) optical communication, multiple component wavelengths of light each carry a communication signal. Each of the multiple component wavelengths of light form a WDM channel. An OADM (optical add-drop multiplexer) is used for WDM signal management. WDM signals are transmitted from location to location using the channels. At a particular location, the signal within each channel is either passed for transmission to another location, or is dropped for local distribution. As signals are dropped, the channels corresponding to those dropped signals are free to accept new signals. The new signals are uploaded into the WDM signal at the same wavelength as the signal that was dropped. Maintaining an active signal in each channel maximizes total bandwidth.
The purpose of wavelength division multiplexing is to carry multiple signals over the same medium at the same time. To accomplish this, a number of channels are used. However, different signals may need to be transmitted to different locations. So, if a given signal is only transmitted a required distance, then that signal is dropped and another signal is added. This maximizes the total bandwidth utilization. In the case where a WDM signal contains signals 1, 2, 3 and 4, two of the signals, 2 and 4, are to be dropped for local distribution at a given location. At the given location, two new signals, 2′ and 4′, containing local information are uploaded. Signal 2′ is modulated at the same wavelength as was signal 2, and signal 4′ is modulated at the same wavelength as was signal 4. Added signals 2′ and 4′ are interlaced with the two passed signals 1 and 3 to form a WDM signal containing signals 1, 2′, 3 and 4′. This process is referred to as an add/drop function.
To perform the add/drop function, the component signals within the WDM signal must first be isolated. Conventionally, a multiplexer/de-multiplexer, such as an array waveguide (AWG), is used to separate the component signals and to direct each component signal to a desired location. Waveguides tend to be expensive, they are typically delicate to set-up and maintain, and often require extensive thermal management.
Once the component signals are isolated, MEMS (MicroElectroMechanical System) devices or tilting mirrors are often used to reflect each component signal in a selectable direction. The component signal is either passed or dropped depending on the selected direction. To select a direction, the mirrors are moved or rotated using some type of mechanical means, for example a piezoelectric or pico-motor. Such mechanical movement produces mirror movements that may be less precise than desired. Mechanical movement also limits the speed by which the mirrors can be moved, and thus limits the speed by which the channels can be added/dropped.
An alternative means to perform the add/drop function is to use a Mach-Zehnder interferometer for each component signal. The Mach-Zehnder interferometer is an amplitude splitting device consisting of two beam splitters. The component signal is split into two portions and each portion is directed along separate optical paths. The two portions are eventually recombined. When recombined the two portions either constructively interfere or destructively interfere depending on whether or not the component signal is to be passed or dropped, respectively. The type of interference is determined by the phase difference between the two portions upon recombination. Changing the optical path lengths of one or both of the two portions can alter the phase difference. A difference between the optical path lengths can be introduced by a slight tilt of one of the beam splitters. To tilt the beam splitter though requires the use of some type of mechanical means, which once again limits speed and precision. Mach-Zehnder interferometers are also expensive and often require extensive thermal management.
What is needed is a method of adding and dropping channels within a WDM signal that is less expensive and simpler to implement and maintain then conventional optical add/drop multiplexers.
What is also needed is a method of adding and dropping channels within a WDM signal that is less expensive and simpler to implement and maintain then conventional optical add/drop multiplexers, and that increases speed and improves precision.